In computing, a line is a unit of organization for text files. A line consists of a sequence of zero or more characters, usually displayed within a single horizontal sequence. The term comes directly from physical printing, where a line of text is a horizontal row of characters. Depending on the file system or operating system being used the number of characters on a line may either be predetermined or fixed, or the length may vary from line to line. Fixed-length lines are sometimes called records. With variable-length lines, the end of each line is usually indicated by the presence of one or more special end-of-line characters. These include line feed, carriage return, or combinations thereof. A blank line usually refers to a line containing zero characters (not counting any end-of-line characters); though it may also refer to any line that does not contain any visible characters (consisting only of whitespace). Some tools that operate on text files (e.g., editors) provide a mechanism to reference lines by their line number.